The increasing penetration of inverter-based resources (IBRs) in power systems has raised many concerns in terms of frequency stability. However, prior art analytical models for system frequency generally cannot precisely account for the impact of voltage dynamics, leading to potentially incorrect results. The main challenge lies in understanding system frequency in the case of non-constant bus voltages, and integrating the voltage dynamics of generation devices at different bus locations into the system frequency model. To address this issue, this article defines the voltage-influenced common-mode frequency (VCMF), serving as a system frequency analysis model considering voltage dynamics. The VCMF is derived through the decomposition of bus frequency responses, leveraging the connection between the consistent part of bus frequencies and the rotational invariance of power flow. The decomposition process introduces voltage dynamics into the system frequency response, represented as a global term that interconnects all devices through the power network. To address the complexity of this global term, an algebraic graph theory-based network partitioning method is introduced. This method effectively divides the globally coupled term into several locally coupled components, making the analysis of the VCMF more manageable. Finally, simulations are provided to validate the proposed methods.